The proposed invention refers to a polymeric composition suitable to adhere and coat different substrates and to conductive coatings prepared from this composition. More particularly the invention refers to a composition having improved temperature resistance and rendering the coating flame-retardant without however deteriorating its conductive and adhesion properties,
Conductive temperature-resistant coatings prepared from the composition of the present invention can be used in various applications including military applications, electronic industry, space applications, measuring equipment operating at high temperatures, e.g. measuring data of volcanic lava, heating equipment, fire simulators, etc.
Conductive polymeric compositions are commonly used for coating of various flat and curved substrates, e.g. heating elements, integrated circuit boards used for carrying electronic components etc.
The main ingredients of a mixture used for preparation of such coatings comprise a component capable to provide conductivity and a binder component.
Polymeric conductive coatings and their various applications are described for example in U.S. Pat. No. 5,484,294, 3,632,440, 5,021,005, WO 9741568.
An example of typical polymeric composition used for preparation of conductive coating can be found in U.S. Pat. No. 5,728,332 disclosing an electrically conductive polymeric coating paste. This composition includes an aromatic solvent, a particulate capable to provide conductivity, a substrate forming elastomer and paraffin,
One typical disadvantage associated with prior art polymeric conductive coatings are their insufficient resistance to high-temperatures and thus limited service life. Practically, most of the prior art coatings, including the above-mentioned conductive polymeric coating paste function properly only at temperatures in the range of 200-400xc2x0 C. At higher temperatures, the prior art coatings deteriorate fast because of oxidation in air and subsequently loose their conductive properties.
The main object of the present invention is to improve temperature resistance of polymeric conductive coatings, without however deterioration of their conductive properties.
Still further object of the invention is to provide a polymeric conductive coatings, having improved fire resistance and capable to function at temperatures higher than 400xc2x0 C.
Yet, another object of the invention is to provide a polymeric conductive coating having improved temperature resistance without substantial increase of cost of its composition.
The above mentioned and other objects of the invention are achieved by virtue of combination of a particulate conductive component with a liquid temperature-resistant component.
It has been unexpectedly revealed that it is very advantageous to use for as temperature resistant component an organic-mineral compound, namely quaternary ammonium silicate (QAS) with modulus 4 or higher. In accordance with the invention, it is important that the ingredients of the composition are thoroughly mixed to obtain a mixture, which does not exhibit sedimentation. In practice, it is advantageous if the particle size of the particulate component does not exceed 100 mesh and does not exceed half of the thickness of the coating layer to be applied to a substrate. The most efficient results in terms of combination of good conductivity and improved temperature resistance are achieved if the conductive component is an amphoteric metal. Possible reason for this could be chemical reaction of amphoteric metal with quaternary ammonium silicate. The product of this reaction is a salt that increases the active surface of the conductive component and renders the coating even more fire-retardant.
It should be also appreciated that besides of the above components the composition may contain some other additives, which usually are required to impart specific properties to the coating. Among such additives can be mentioned fillers, pigments, etc. It is possible to use known commercially available inorganic metallic or non-metallic fillers, e.g. nickel, cobalt, iron, graphite, etc. As suitable pigment, one can use titanium dioxide, iron oxide, chromium oxide, etc.
It has been also found that polymeric conductive coatings of the invention are most suitable for applying on heating elements when their conductivity is relatively low and for applying on electronic circuit boards when their conductivity is relatively high.